1. Introduction
This invention relates to a photoresist composition comprising a light-sensitive component admixed with a binder that is a novel novolak resin formed from one or more phenols and one or more aromatic aldehydes or a blend of said novolak resin with another resin typically used in a photoresist composition including conventional novolak and other phenolic resins.
2. Description of the Prior Art
Photoresist compositions are well known in the art and described in numerous publications including DeForest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative-acting resist is initially a mixture which is soluble in its developer, but following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive-acting resists work in the opposite fashion, light exposure making the resist soluble in developer.
Positive-working photoresists are more expensive than negative-working photoresists but are capable of providing superior image resolution. For example, the positive-working photoresist described above can be developed to yield relief images with a line width as low as one micron or less. In addition, considering the cross section of a photoresist image, the channels formed in the resist by development have square corners and sidewalls with only minimal taper.
The positive-working resists comprise a light sensitive compound in a film-forming polymer binder. The light sensitive compounds, or sensitizers as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and are described by DeForest, supra, pages 47-55, incorporated herein by reference. These light sensitive compounds, and the methods used to make the same, are all well documented in prior patents including German Pat. No. 865,140 granted Feb. 2, 1953 and U.S. Pat. Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122 and 3,106,465, all incorporated herein by reference. Additional sulfonic amide sensitizers that have been used in the formulation of positive-acting photoresists are shown in U.S. Pat. No. 3,637,384, also incorporated herein by reference. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Pat. No. 2,797,213, incorporated herein by reference. Other positive-working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive-working photoresist for deep U.V. lithography is Meldrum's diazo and its analogs as described by Clecak et al, Technical Disclosure Bulletin, Volume 24, Number 4, September 1981, IBM Corporation, pp. 1907 and 1908. An o-quinone diazide compound suitable for laser imaging is shown in U.S. Pat. No. 4,207,107. The aforesaid references are also incorporated herein by reference.
A class of negative resists comprising a negative-acting sensitizer in a polymer binder is described by Iwayanagi et al, IEEE Transactions on Electron Devices, Vol. ED-28, No. 11, November, 1981, incorporated herein by reference. The resists of this reference comprise an aromatic azide in a phenolic binder. It is believed that these resists are first disclosed and claimed in U.S. Pat. No. 3,869,292, also incorporated herein by reference. Additional aromatic azide sensitizers are disclosed by DeForest, supra, and U.S. Pat. Nos. 2,940,853 and 2,852,379, incorporated herein by reference.
The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali soluble phenol formaldehyde resins known as the novolak resins. Photoresists using such polymers are illustrated in U.K. Pat. No. 1,110,017, incorporated herein by reference. These materials are the product of reaction of a phenol with formaldehyde under conditions whereby a thermoplastic polymer is formed with a glass transition temperature of about 100.degree. C. Novolaks with glass transition temperatures in excess of 100.degree. C are known but are not generally used in photoresist formulations because they are expensive and involve extraction of low molecular weight fractions.
Another class of binders used with both the negative-acting aromatic azides and the positive acting o-quinone diazides are the homopolymers and copolymers of vinyl phenol. Photoresists of this nature are disclosed in U.S. Pat. No. 3,869,292, supra. It is believed that photoresists using binders of polymers formed from vinyl phenols have not found extensive use in commerce.
In the prior art, the above described positive resists using novolak resins as a binder are most often used as masks to protect substrates from chemical etching in photo-engraving processes. For example, in a conventional process for the manufacture of a printed circuit board, a copper-clad substrate is coated with a layer of a positive working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine line image resolution.
Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared by the development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer and must be capable of very fine line image resolution.
Recent developments in photoresist technology involve processes where high temperatures are encountered. For example, a recent development in the fabrication of semiconductors substitutes dry plasma etching for wet chemical etching to define a circuit. Plasma etching provides advantages over wet chemical etching in that it offers process simplification and improves dimensional resolution and tolerance. However, the demands on the resist are significantly greater when using plasma etching. For both wet etching and plasma etching, the resist must adhere to the substrate and must be capable of fine line image resolution. For plasma etching, in addition to these properties, the resist must often be capable of withstanding high temperatures without image deformation and without eroding as plasma etching generates high temperatures at the wafer surface.
The above described prior art positive-working resists provide good resistance to chemical etchants and fine line image resolution. However, they soften and begin to flow at temperatures somewhat in excess of about 120.degree. C. This can result in image distortion and poorer image resolution.